How do I tell when it's okay to transition to a new language?

If you want to learn multiple languages, knowing when and how to transition to a new language is important. It's recommended to only focus on one language at a time. To know how to transition, or when to transition to a new language without "losing" the old one, consider the following:

In terms of the 'when', we'd suggest only transitioning once you feel like you can comfortably think in the language. Maybe you don't converse fluently about every possible topic, but still, you should notice that you can hold some conversations without translating in your head. At that point, you've built something pretty robust, and while you still may forget things, it'll be a lot easier to bring the language back without too much work. Perhaps even better than that point, when you can comfortably watch TV in your target language without subtitles, you may be ready.

Regarding the 'how', generally, we'd stop doing Anki flashcards for our old language and start going full tilt on the new one. To reduce forgetting the old one, what you might do right now is chat with your old tutor on iTalki every 2 weeks. That seems to be enough to keep it active (and since it's iTalki, it's like $5/hour, so it's not breaking the bank). You no longer need intense vocabulary/grammar training; you can just chat about random stuff, and it works quite well. Alternatively, you can drop the old language altogether, then bring it back using videos.